We basically have 2+x+y on each side. The difference is the use of parenthesis in different places. With addition, we can add in any order we want and that includes grouping terms as well. For example (2+3)+5 = (5)+5 = 5+5 = 10 2+(3+5) = 2+(8) = 2+8 = 10
So that's why in general (2+x)+y = 2+(x+y) is true for all real numbers x and y.
